1. Field of the Invention
The present invention pertains to variable frequency oscillators (VFOs) and, more particularly, to a VFO having an increased tunable frequency range.
2. Description of the Related Art
Currently employed variable frequency oscillators (VFOs) are manufactured to achieve a desired target frequency (F) and provide for a limited tuning range relative to the circuit operating frequency to accommodate for shifts caused during manufacture or as required for operation. This tuning range allows the operating frequency to be adjusted downward from an actual frequency value to a value closely proximate the target frequency. Such VFOs include a tank circuit and an active device, such as a transistor, connected to a control box for adjusting the VFO frequency.
A known prior art VFO circuit 5 is shown in FIG. 1 having a tank circuit 10 connected to an active device 20. The tank 10 employs an LC oscillator circuit having a tank inductor L with a fixed inductance connected in parallel with a tank capacitor C having a fixed capacitance. A varactor having a variable capacitance value is provided for tuning the VFO frequency by adjusting the value of the varactor capacitance. Since the varactor is connected in parallel with tank capacitor C, the overall capacitance of the VFO circuit will be increased by a maximum value of the maximum varacator capacitance (.DELTA.C), thereby reducing the frequency F of the tank 10 by a maximum amount (.DELTA.f). In addition, a switchable capacitor C' is connected in parallel to the load capacitor C by a switch S1 for providing a stepped increase of the overall capacitance of the tank 10 and, therefore, a stepped decrease (.DELTA.f') in the resulting frequency. For example, when switch S1 is closed, thereby incorporating capacitor C' into tank circuit 10, the overall capacitance of the circuit will be C+C'+.DELTA.C, resulting in a frequency of F-.DELTA.f'-.DELTA.f. As such, currently employed VFO circuits, such as the circuit 5 of FIG. 1, are limited in that they only provide for the VFO frequency to be reduced downward from a set frequency, as opposed to also or alternatively being increased above the set frequency, thereby limiting the versatility and tuning range of these known VFO circuits. Although it may be possible, to an extent, to over-design tank circuits having an increased maximum frequency with an increased downward tuning range--in an attempt to avoid the need for an upward tuning range--such an alternative is limited. This is because the incorporation of multiple switches S1 for the required multiple switchable capacitors needed to increase the downward tuning range add detrimental parasitic capacitance to the tank.